The influence of surface wave on water exchange in the Bohai Sea

Highlights

• A Coupled model is used for analyzing the water exchange processes in the Bohai Sea.

• The discrepancy between particle model and dye model confirms circulation structure.

• Ocean waves can enhance water exchange rate, especially the surface layer.

• Various aspects of waves on the water exchange are examined.

Abstract

A three-dimensional wave-current coupled modeling system has been applied to analyze the water motions for the Bohai Sea and its adjacent waters. Two different methods of estimating the water exchange through the Bohai Strait have been employed, consisting of particle tracking and passive dye. The objectives of this study are to account for the surface wave role in the water exchange processes between the Bohai Sea and the outer waters, to test the response of the flushing characteristics of the Bohai Sea to different aspects of wave actions, and to obtain a quantitative and qualitative estimate of the half-life time under different experiment conditions. By comparing the simulations of water exchange with or without wave-current interactions, we find that waves can improve the Bohai Sea vertically mean water exchange capability, with more obvious enhancements for the surface layer where concentrated actions function. A series of numerical experiments indicate that turbulent kinetic energy from wave dissipation is the major positive influence, while wave-dependent surface stress, radiation stress, and Stokes drift have a minor effect. These results suggest that wave-driven mixing should be considered in the Bohai Sea water exchange processes, preferably using a half-life model based on concentration advection-diffusion model.

Introduction

Water exchange between shallow sea and the open ocean is a key process which affects the distribution and fate of nutrients, pollutants and sediments. Since there are various dynamic phenomena in coastal seas, different definitions and methods for the calculation of water exchange times have been presented. The flushing time concept is widely employed, defined as the time for a concentration to decrease to 1/e of its initial concentration (Takeoka, 1984; Andutta et al., 2013), analogous to the half-life time, i.e. time needed to reduce the concentration to 50% of that (Luff and Pohlmann, 1995; Liu et al., 2004). Due to the equivalency between tracking particles and solving a mass transport equation for a conservative substance (Dimou, 1992), the Lagrangian method is also capable for calculating the residence time (Marinone et al., 2011), which is the time taken for a water particle to cross the specified boundary. It remains unsolved faced with how to reasonable distribution of particle distribution in space, especially for the unstructured mesh.

Huthnance (1995) gave preliminary assessment of various influential processes contributions to water exchange at the ocean margin according to scale analysis. Tides, winds and density differences are the principal agents for the exchange of water between the shallow coastal waters and the outer offshore zone (Guyondet et al., 2005; Safak et al., 2015). Delpey et al. (2014) adopted a three-dimensional wave-current model to investigate wave-induced circulation in a small estuarine bay and its influences on freshwater exchange with the inner shelf, concluding that the effect of the surf zone current on the bay flushing time is larger than that of wind. However, wave-driven significance (dominant in the surf zone) in the marginal sea has not yet been fully analyzed on the basis of the estimation that the time scale of water renewal time is much longer than the wave period. Recent studies suggest that smaller scales surface waves can yield a series of significant contributions to water dispersion (Jones and Monismith, 2008), one of which is the Stokes drift. McWilliams and Restrepo (1999) pointed out that wind-driven current in the presence of Stokes drift should be modified by the addition of so-called vortex forces and wave-induced advection. Other effects from waves to currents include radiation stress gradients, wave dissipation as a source term in the energy equation, the effective surface drag coefficient varied with the wave age, and the modified bottom frictions (Davies and Lawrence, 1994). Jordá et al. (2007) stated that the main effect of waves on current structure is through the modification of the wind drag, and the Stokes drift also played a role due to its spatial and temporal characteristics. Numerical model demonstrated that wave breaking was overwhelmingly dominant factor for the circulation and flushing of Ningaloo throughout the whole year (Taebi et al., 2012). The inclusion of radiation stress in the momentum equation could produce an excess flow in the same order of magnitude as the one produce by considering a sea-state dependent wind stress formulation (Osuna and Monbaliu, 2004). When the effect of bottom friction are taken account may act equally strong on the mean current, and the friction-dependent wave-induced mean Eulerian current is even more larger than the Stokes drift for idealized shelf topography (Weber and Drivdal, 2012). By using wave-current model to investigate water exchanges, Delpey et al. (2014) regarded wave as a positive factor, while Malhadas et al. (2010) gave an opposite perspective of residence time in Óbidos Lagoon. The distinct contradiction reflects that a comprehensive analysis of these aspects of wave motion on water exchange processes is required to clarify their relative importance.

The Bohai Sea is a shallow, semi-enclosed marginal sea of the West Paciffic Ocean, with east-west width about 300 km and north-south length about 550 km and an area of 77,000 km². Surrounded by the mainland and Liaodong peninsula, it has the sole passage in the east connecting to the outer Yellow Sea, the Bohai Strait. The Bohai Sea is shallow with 95% of the area having a depth of <30 m, and an average water depth of 18.7 m. Many potential or actual sources of pollution are located around the Bohai Sea, such as Hongyanhe nuclear power plant, dotted harbors, large industries, oil platforms, waste water treatment plants, etc. As a result, the Bohai Sea is a particularly vulnerable area due to the different types of potential spills and to its semi-enclosed nature. Therefore, new scientifically based tools for adequate response and coastal management are specially needed. The time scale for the renewal of the Bohai water masses has been an issue with different views, ranging from a few months to several decades (Qiao, 2012). It is normal for the large-scale flows vary inter-annually, and the shorter time scale circulation also shows large variability (Huthnance et al., 2002). While a wide range of hydrodynamic studies have been carried out to investigate the Bohai Sea environmental problems, there is less information on comprehensively evaluation of water exchange timescales due to various dynamic factors.

In the Bohai Sea the tidal movement from the Yellow Sea propagates through the Bohai Strait is one of the prominent hydrodynamic processes. Huang et al. (1999) investigated baroclinic circulation in the Bohai Sea due to seasonal density variations, finding that inflow was mainly confined to a narrow passage in the northern Bohai Strait, but the outflow was weak in the rest of the strait. Based on half-life water quality model, Wei et al. (2002) pointed out that there were evident regional variations for exchange ability distributed in different areas. Laizhou Bay has the highest exchange ability, while Liaodong Bay, especially in the northwest part, has the lowest one. Hainbucher et al. (2004), using a three-dimensional, prognostic baroclinic hydrodynamics model, calculated turnover times and taken into account comprehensive environmental conditions, such as river runoff, wind, tides and thermohaline effects except surface waves. By calculating water age of Yellow River in the Bohai Sea, Liu et al. (2012) found that tidal forcing was the dominant role for water renewal in the Bohai Sea, followed by the wind forcing. Wave actions significantly impact on the spatial distribution of suspended sediment in the Bohai Sea (Wang et al., 2014) and temperature pattern in the Yellow Sea (Zhang et al., 2011), so it is of considerable reason to investigate their impacts on water exchange processes with the outer open waters.

This paper presents results of a numerical study undertaken at the Bohai Sea, China, with the purpose to evaluate the role of surface waves in different aspects on the Bohai water exchange capacity. The paper is structured as follows: In Section 2 the water exchange model framework is described, which employs a third-generation wind wave model (SWAN version 40.85) and a three dimensional hydrodynamic model (SELFE version 3.1dc) to offer transport filed incorporating wave-current interactions. The influences of the variation of the surface wave on the water half-life time and concentration of passive material are examined in Section 3, and the conclusions are presented in Section 4.